The present invention relates to a process for the preparation of acrolein and or acrylic acid from propane, in which the propane is subjected, in a first reaction stage, to a partial oxydehydrogenation with molecular oxygen under homogeneous and/or heterogenous catalysis to give propene and the propene-containing product gas mixture formed in the first reaction stage is then used in at least one further reaction stage for the preparation of acrolein and/or acrylic acid by gas-phase catalytic propene oxidation.
Acrolein and acrylic acid are important intermediates which are used, for example, in the preparation of active ingredients and polymers.
The process predominantly used at present on an industrial scale for the production of acrolein and/or acrylic acid is the gas-phase catalytic oxidation of propene (e.g. EP-A 575 897), the propene being produced predominantly as a byproduct of ethylene production by steam cracking of naphtha.
Since the other fields of use of propene, for example the preparation of polypropylene, are constantly expanding it would be advantageous to have an industrially usable, competitive process for the preparation of acrolein and/or acrylic acid, whose raw material base is not propene but, for example, the propane naturally occurring in large quantities as a natural gas component.
U.S. Pat. No. 3,798,283 discloses that propane can be oxydehydrogenated homogeneously to propene in the presence of molecular oxygen at elevated temperatures. Suitable oxygen sources are both pure oxygen and mixtures of oxygen and inert gas.
DE-A 20 58 054 and DE-A 1 95 30 454 disclosed that the oxydehydrogenation of propane to propene can also be carried out under heterogeneous catalysis.
U.S. Pat. No. 3,161,670, EP-A 117 446 and DE-A 33 13 573 relate to processes for the preparation of acrolein and/or acrylic acid, in which propane is first dehydrogenated under heterogeneous catalysis in the absence of oxygen to give propene.
The propene-containing product mixture is then subjected to a gas-phase oxidation under heterogeneous catalysis. However, the disadvantage of this procedure is that the catalyst required for the nonoxidative dehydrogenation of the propane is relatively rapidly deactivated by carbon deposits and therefore has to be regenerated frequently. A further disadvantage of this procedure is the hydrogen formation associated with the nonoxidative propane dehydrogenation.
It is true that DE-A 33 13 573 mentions the basic possibility of coupling oxidative dehydrogenation of propane to propene with subsequent propene oxidation under heterogeneous catalysis. However, it does not contain more detailed information on carrying out such a process.
EP-A 293 224, U.S. Pat. Nos. 5,198,578 and 5,183,936 state that a high proportion of N2 in the diluent gas of the catalytic gas-phase oxidation of propene to acrolein and/or acrylic acid is disadvantageous. EP-A 293 224 furthermore suggests combining the oxidative dehydrogenation of propane to propene and the catalytic gas-phase oxidation of propene for the preparation of acrolein and/or acrylic acid with one another.
In Catalysis Today 13, (1992), 673 to 678, Moro-oka et al. combine a homogenous oxidative dehydrogenation of propane to propene with a subsequent oxidation of the dehydrogenation product mixture under heterogeneous catalysis to give acrolein and/or acrylic acid in laboratory experiments. The corresponding combination of processes is recommended by Moro-oka et al. in Applied Catalysis, 70 (2), (1991), 175 to 187. In accordance with the recommendation of EP-A 293 224, of U.S. Pat. No. 5,198,578 and of U.S. Pat. No. 5,183,936, Moro-oka et al. use either pure molecular oxygen or air enriched with oxygen as the oxygen source for the oxydehydrogenation stage in all cases. For the latter case, Moro-oka suggests no separation at all, in the further course of his process, of nitrogen introduced into the process.
CN-A 110 5352 likewise discloses a homogenous oxidative dehydrogenation of propane to propene with a subsequent oxidation of the dehydrogenation product mixture under heterogeneous catalysis to give acrolein and/or acrylic acid. Since CN-A 110 5352 relates to a procedure to be carried out on industrial scale, CN-A 110 5352, following the recommendation of EP-A 293 45 224, of U.S. Pat. No. 5,198,578 and of U.S. Pat. No. 5,183,936, uses exclusively pure molecular oxygen as the oxygen source.
WO 97/36849 relates to the combination of a catalytic oxidative dehydrogenation of propane to propene with a subsequent oxidation of the dehydrogenation product mixture under heterogeneous catalysis to give acrolein and/or acrylic acid in an industrial embodiment.
Although WO 97/36849 does not rule out the use of nitrogen-containing oxygen (e.g. air) as a source of the molecular oxygen required for the oxydehydrogenation, it advises against such a use. Moreover, for a continuous procedure with recycled gas for suppressing an undesired concentration of disadvantageous components of the reaction gas mixture, WO 97/36849 suggests merely a purge of recycled gas and no separation of components from the recycled gas.
For cost-efficiency reasons, essentially only air is suitable as a starting material for the molecular oxygen source for industrial gas-phase oxidation.
Against this background, the abovementioned procedures are disadvantageous in that, owing to the similarity of O2 and N2, the sole measure of a prior nitrogen/oxygen separation, starting from air, for the preparation of pure oxygen or of an air depleted in nitrogen for limiting the nitrogen content in a subsequent oxidation of the propene contained in the dehydrogenation product mixture is very energy-consumptive.
It is an object of the present invention to provide a process for the preparation of acrolein and/or acrylic acid from propane, in which the propane is subjected, in a first reaction stage, to an oxydehydrogenation with molecular oxygen under homogeneous and/or heterogeneous catalysis to give propene and the propene-containing product gas mixture formed in the first reaction stage is then used in at least one further reaction stage for the preparation of acrolein and/or acrylic acid by gas-phase catalytic propene oxidation, and in which the nitrogen content in the propene oxidation stage is limited in a less energy-consumptive manner than in the prior art.